1. Field of the Disclosure
The present disclosure relates to an external cavity surface emitting laser, and more particularly, to an end pumping external cavity surface emitting laser in which pumping beam is recycled using a pumping beam reflection layer to increase absorption of the pumping beam by an active layer.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) oscillates in a single longitudinal mode of a very narrow spectrum and emits a beam having a small radiation angle. VCSELs can be integrated easily with other devices, but the output of the VCSELs is low.
A vertical external cavity surface emitting laser (VECSEL) is a high output laser with the above-described advantages of the VCSEL. The VECSEL has an external mirror instead of an upper mirror, resulting in an increased gain region, and can thus output several to dozens of watts of light.
FIG. 1 is a schematic view of a VECSEL 10. The VECSEL 10 is a front pumping laser in which light is pumped by a pump laser 15 which is disposed obliquely in the front of the VECSEL 10. As illustrated in FIG. 1, the VECSEL 10 includes a heat sink 11, a Distributed Bragg Reflector (DBR) layer 13 and an active layer 14 sequentially stacked on the heat sink 11, an external mirror 17 that faces the active layer 14 and is separated a predetermined distance from the active layer 14, and a pump laser 15 disposed obliquely toward the top surface of the active layer 14. A heat spreader 12 may be further formed on the top surface of the active layer 14 to spread the heat generated by the active layer 14, and a second harmonic generation (SHG) crystal 18 which doubles the frequency of the light output may be placed between the active layer 14 and the external mirror 17. Also, the VECSEL 10 includes a collimating lens 16 that collimates the pumping beam emitted from the pump laser 15. For instance, the active layer 14 may have a multiple quantum well structure having a resonant periodic gain (RPG) structure and is excited by the pumping beam to emit light with a predetermined wavelength λ2. The pump laser 15 inputs a pumping beam having a shorter wavelength λ1 than the wavelength λ2 of the light emitted from the active layer 14 to excite the active layer 14.
In the above described configuration, a pumping beam with a relatively short wavelength λ1 emitted from the pump laser 15 is incident on the active layer 14, and the active layer 14 is excited to emit light with a predetermined wavelength of λ2. The emitted light is reflected repetitively between the DBR layer 13 and the external layer 17 through the active layer 14. Thus, a portion of the light amplified in the active layer 14 is output to the outside via the external mirror 17. When the SHG crystal 18 is interposed between the active layer 14 and the external mirror 17, for example, light in the infrared region emitted from the active layer 14 is converted into visible light and then output.
FIG. 2 is a schematic view of a conventional VECSEL 20 using end pumping. In the VECSEL 10 using front pumping illustrated in FIG. 1, the incident surface of the pumping beam in the active layer 14 and the emission surface of the output light are the same. That is, a pumping beam is incident through the top surface of the active layer and the output light is emitted through the top surface of the active layer 14. On the other hand, as illustrated in FIG. 2, in the VECSEL 20 using end pumping, a pumping beam is incident through the lower surface of the active layer 23 and the output light is emitted through the top surface of the active layer 23 to an external mirror 25 facing the active layer 23. For example, a DBR layer 22 and an active layer 23 are stacked sequentially on a light transmissive heat spreader 21 which is formed of diamond or silicon carbide (SiC), and a pump laser 24 faces the active layer 23 with the light transmissive heat spreader 21 interposed therebetween. Accordingly, a pumping beam emitted from the pump laser 24 passes through the light transmissive heat spreader 21 and is incident on the lower surface of the active layer 23. Such end pumping enables easy assembly of the VECSEL and reduces light loss since a pumping beam is perpendicularly incident on the active layer.
However, in the conventional VECSEL, a pumping beam emitted from the pump laser may not be completely absorbed in the active layer, and a portion of the pumping beam is dispersed by the heat sink or passes through the active layer and then emitted. In VECSELs using front pumping, a portion of the pumping beam which is not completely absorbed in the active layer passes through the DBR layer and is wasted. Also, in VECSELs using end pumping, a portion of the pumping beam which is not absorbed by the active layer is emitted through the top surface of the active layer. Accordingly, conventional VECSELs cannot efficiently use the energy of the pumping beam, and thus have low efficiency.